Overexposure to ultraviolet radiation (UVR) is a recognised health hazard. Acute effects arising from short-term exposure include sunburn (erythema) and photo-conjunctivitis. Long-term exposure can lead to chronic conditions such as photoaging, skin cancer and cataracts. Incident UVR levels are usually measured based on the UV index system, where a UVI value of 6 is typical of summer's day in the UK and equivalent to an effective UV irradiance of 150 mW m−2. The minimum erythemal dose, MED, is the minimum amount of UVR likely to cause erthema and is (i.e. MED=1) ca. 250 J m−2 for most Caucasians, i.e. skin type II. Thus, the latter would be likely to sunburn after only 28 min on a summer's day in the UK. Despite the much greater awareness of the general public of the potential dangers of UVR overexposure, the number of attributed cases of skin cancer continues to rise such are the continued attractions of possessing a suntan. In the UK currently ca. 50,000 people develop skin cancer pa, 8000 of which are malignant leading to ca. 2000 deaths pa.
The major problem with sunburn is that the usual signs of skin burning and damage are delayed; often taking 4-8 h to appear. Thus, it is would be useful to have an easily read real-time indicator which shows when an MED=1 value has been reached for any particular skin type and it is time to cover up from the sun. There are several notable UV dosimeters on the market, including: SolarSafe, SunSignals and SunCheck. Most utilise a single gradual colour change which makes it hard to identify useful stages in the development of sunburn, such as MED=0.5 and 1.
One method to detect irreversibly UV light is to combine UV-driven acid release agents, HA, including chloral hydrate (CH) and iodonium and sulphonium salts with the deprotonated form of a pH indicator (D−). The two key processes can be summarised as follows:HA→H++A−  (1)D−+H+→DH  (2)
The result is that with UV irradiation time the deprotonated form of the dye, D, is converted into its conjugate acid form, DH and this is associated with a colour change since, given the nature of pH indicators, D− is very differently coloured to DH. For example, Thymol Blue (TB−) is yellow whereas TBH is red. However, such a system is generally understood to result in a prompt or substantially instantaneous colour change in response to UV stimulation, which does not therefore easily allow a user to determine a relative UV dose, or a UV dose over time.
Papers published by Abdel-Fattah et al deal primarily with such UV dosimeters based on the chloral hydrate acid release agent, supported on a glass substrate1,2.
U.S. Pat. No. 4,829,187 covers a UV dosimeter which uses a UV driven acid release agent in an indicator layer supported on a polymer or paper substrate, with a UV blocking layer on top. Alkyl halides may be used as the HA in this system.
U.S. Pat. No. 5,117,116 describes a plastic (PVC) device, attached to the skin with a pressure sensitive adhesive, which supports an indicator label just above the surface of the skin. The preferred photosensitive material in this device is an oxazolidine-dione compound but it is noted that other materials can be used, such as a combination of iodonium salts and pH sensitive dyes.
U.S. Pat. No. 6,504,161 describes “Sun Signals”™ device distributed by Sun Health Solutions and is similar in design to the system described in U.S. Pat. No. 5,117,116. This device relies on the change in colour of a pH sensitive dye—including any from the Aldrich Handbook of Stains, Dyes and Indicators—caused by the release of acid from an alkyl halide. The dye and halide are contained in a polymer support. The single colour change in this system is designed to disguise or reveal a graphic pattern depending on the manufacturer's preference.
Finally, US 2002/00298 describes a polymer layer applied directly to the skin which contains a UV sensitive agent. This agent is primarily taken from classic photochromic groups—spirooxazines, spiropyrans, fulgides, fulgimides, bisimadizoles, viologen derivatives and combinations thereof—but it is mentioned that a system incorporating iodonium salts, i.e. a UV driven acid release agent, may be used.
Nevertheless, all of the above devices describe single colour change, prompt, UV indicators/dosimeters; the only guide to the UV dose received being the depth of the colour change. Moreover, they all disclose devices which provide little information with regards to the level of the UV dose received nor a very clear indication when a MED=1 level has been achieved.
It is amongst the objects of the present invention to obviate and/or mitigate at least one of the aforementioned disadvantages.
It is an object of the present invention to provide a device/method which utilises a delayed UV indicator, which can optionally be used in conjunction with a prompt UV indicator to highlight key, different UV doses, such as 0.5, 1, and 2 MED.